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As far as my understanding goes, custom diet plan is the diet plan which is customized or tailor made to suit one's need. You go with a certain goal like losing weight or managing some disease like diabetes etc. Various medical tests and reports are taken into view and other parameters like your life style, your food preference, your body's requirement and also your psychological, emotional and spiritual inclinations are taken in to account. Then a diet plan is decided to suit your needs. The saying that one size does not fit all holds true to any diet.
Custom diet plan essentially means having a plan for you after taking into consideration of your health, physical state & your target requirement & giving you a plan that suits your lifestyle & your food habits.
I am not sure where did you read about it or missed the term but the fact is that this custom diet plan is all about losing or gaining weight. In most cases people follow custom diet plan to lose weight.... so there!